The present invention generally relates to fuel-fired heating appliances, such as furnaces, water heaters and boilers and, in a preferred embodiment thereof, more particularly relates to apparatus and methods for reducing NO.sub.x emissions generated by the combustion systems in such appliances.
Nitrogen oxide (NO.sub.x) emissions in fuel-fired heating appliances, such as furnaces, water heaters and boilers, are a product of the combustion process, and are formed when the combustion reaction takes place at high temperature conditions typically encountered in such heating appliances. NO.sub.x emissions became an environmental issue in the late 1960's and early 1970's due to their detrimental role in atmospheric visibility, photochemical smog and acid deposition. Regulations in the subsequent decade led to significantly reduced amounts of NO.sub.x emissions.
Current SCAQMD (South Coast Air Quality Management District) regulations for residential furnaces and water heaters limit NO.sub.x emissions to 40 ng/j of useful heat generated by these types of fuel-fired appliances. Growing environmental concern is leading to even more stringent regulation of NO.sub.x emissions. For example, regulations currently being proposed by SCAQMD for water heaters and boilers limit NO.sub.x emission levels to 30 ppm at 3% oxygen, which is approximately 20.5 ng/j for middle efficiency water heaters and boilers. Conventional fuel-fired appliance combustion systems are not currently capable of meeting these more stringent limitations. For example, a typical in-shot burner system typically employed in these types of fuel-fired appliances produces NO.sub.x emission levels in the range of from about 50 ng/j to about 70 ng/j.
One technique currently used to lower NO.sub.x emissions in fuel-fired heating appliances is to position a heat absorbing flame insert within the burner flame path for "quenching" purposes. The resulting lowered combustion flame temperature results in lowered NO.sub.x emission rates. For example, as shown in U.S. Pat. No. 5,146,910, flame cooling can be achieved by placing an insert within the burner flame zone. The insert receives heat from the flame and radiates heat away to thereby cool the flame. Using this quenching technique, gas furnaces with flame inserts are now in commercial production and have NO.sub.x emission rates of somewhat less than about 40 ng/j.
Flame insert methods are relatively easy and inexpensive to implement. However, NO.sub.x reduction achieved by existing flame inserts is rather limited because conventional flame insert designs are operative solely through a flame cooling mechanism and, for a given combustion system, only limited flame cooling can be realized without jeopardizing the combustion process itself. Due to this practical limitation, existing flame inserts are able to reduce NO.sub.x emissions to about 30 ng/j--considerably short of the proposed emission limitation set forth above.
Some advanced combustion systems such as infrared/porous matrix surface burners, catalytic combustion and fuel/air staging could reach a very low NO.sub.x emission level in compliance with these proposed emission standards, but these methods tend to be quite expensive and usually require extensive system modification. Accordingly, they are not suited for retrofitting existing combustion systems to achieve the desired substantial reduction in system NO.sub.x emissions.
From the foregoing it can be seen that it would be highly desirable to provide improved NO.sub.x reduction apparatus, for use in fuel-fired heating appliances of the type generally described above, which will enable the meeting of the proposed NO.sub.x emission standards in a cost-effective manner and is suitable for retrofitting existing combustion systems with the reduction apparatus. It is accordingly an object of the present invention to provide such improved NO.sub.x reduction apparatus.